Water heating apparatus, especially for pools

ABSTRACT

A water heater for heating water includes a burner assembly ( 54 ) for generating a flow of hot gas. The burner assembly ( 54 ) includes a gas burner ( 52 ) and means ( 64 ) adjustable to determine a heat output of the burner assembly ( 54 ). The water heater further includes a heat exchanger assembly ( 20 ) for transferring heat from gas to water flowing therein. The heat exchanger assembly ( 20 ) has a higher temperature zone ( 110 ) and a lower temperature zone ( 100 ). The water heater is arranged to convey the flow of hot gas to the higher temperature zone ( 110 ) and in turn to the lower temperature zone ( 100 ). The water heater further includes ducting ( 22, 23 ) to conduct said flowing water to and from said heat exchanger assembly ( 20 ), means ( 40 ) to monitor the temperature of the hot gas intermediate the higher temperature zone ( 110 ) and the lower temperature zone ( 100 ), and control means ( 70 ) responsive to the temperature monitoring means ( 40 ) to modulate the heat output of the burner assembly ( 54 ) whereby to maintain the monitored temperature within a predetermined range so as to substantially prevent or minimise condensation of vapour from the hot gas in the higher temperature zone ( 110 ). There is also disclosed a modular heat exchanger apparatus ( 20 ) including like headers ( 34 ) for redirecting fluid within respective modules ( 24 ) and for interconnecting modules ( 24 ). There is also disclosed a water heater having a condensate duct ( 84 ) to direct condensate into the water for chemically treating the water.

FIELD OF THE INVENTION

This invention relates generally to water heating equipment, but hasparticularly useful application to pool and spa heaters. Respectiveaspects of the invention are concerned with a novel configuration ofwater heater, with a heat exchanger arrangement useful in pool and spaheaters, and with a practical use for the condensate that is aby-product of certain types of water heaters.

Throughout this specification, the term “pool” includes in its ambit anykind of confined water body in which humans can be immersed, includingspas, swim spas and Japanese-style immersion tubs.

BACKGROUND OF THE INVENTION

Pool heating is conventionally effected either by circulating the poolwater through solar panels, typically roof-mounted, or by means ofgas-fired water heaters. Water heaters for this purpose are designed toheat a continuous flow of water circulated from the pool to a targettemperature in a range comfortable for swimming, and so the requirementsdiffer considerably from, for example, hot water services, where astatic body of water is heated in a tank to a relatively hightemperature, and hydronic central heating systems, where a flow of wateris heated but the total volume of water is much less and the targettemperature significantly higher.

Conventional pool heaters typically have a gas burner assembly thatgenerates a hot gas flow employed to heat the water as it traversesmultiple tubes in a heat exchanger. Because of the relatively high watervolume and relatively low water temperature, these systems must addressthe problem of condensation in the gas stream as it passes among theheat exchanger tubes: the condensate is slightly acidic because of theuptake of combustion products, and is therefore a corrosive by-product.A popular material for the heat exchanger tubes is cupronickel, which isespecially susceptible to corrosion by the condensate.

Modern pool heater controllers advantageously receive a measurement ofthe pool water temperature, and thermostatically control the operationof the heater, and as the pool water temperature approaches a desiredtemperature, modulate the heater down to a very low power level tomaintain the pool water temperature without noticeable stopping andstarting of the heater. It has been discovered that operation at thisvery low power level results in low flue temperatures such thatcondensation and corrosion is particularly problematic.

The common approach to corrosion prevention is to design the heater sothat at the maximum water flow condition, minimum water temperature anda predetermined gas flow rate the temperature in the heat exchangerremains above the dew point temperature at which condensation begins tooccur. Water flow is typically reduced through the heat exchanger bydiverting a proportion of the flow via a bypass. This approach placeslimits on the efficiency achievable with the overall heaterconfiguration.

Two publications that illustrate known approaches to corrosionprevention are European patent publication 0226534 and Japanesepublished (Kokai) application 11351559.

It is an object of the invention, at least in one or more aspects orapplications, to improve the efficiency of pool heater systems.

SUMMARY OF THE INVENTION

The invention involves, in a first aspect, a different approach totemperature management in the heat exchanger, and, in a second aspect,the adoption of a two-part heat exchanger whereby condensate is anacceptable by-product. In a third aspect, the invention proposesrecycling of the condensate for usefully treating the pool water.

The invention accordingly provides, in its first aspect, a water heaterfor heating water, including:

-   -   a burner assembly for generating a flow of hot gas, which burner        assembly includes a gas burner and means adjustable to determine        a heat output of the burner assembly;    -   a heat exchanger assembly for transferring heat from gas to        water flowing therein, wherein the heat exchanger assembly has a        higher temperature zone and a lower temperature zone, the water        heater being arranged to convey the flow of hot gas to the        higher temperature zone and in turn to the lower temperature        zone;    -   ducting to conduct said flowing water to and from said heat        exchanger assembly;    -   means to monitor the temperature of the hot gas intermediate the        higher temperature zone and the lower temperature zone; and    -   control means responsive to said temperature monitoring means to        modulate the heat output of the burner assembly whereby to        maintain the monitored temperature within a predetermined range        so as to substantially prevent or minimise condensation of        vapour from the hot gas in the higher temperature zone.

By modulating the heat output of the burner the monitored temperaturecan be maintained within a pre-determined range without reducing thevolume of water flowing through the heat exchanger thereby improvingefficiency. It is desirable to minimise said monitored temperature inorder to maximise efficiency.

Preferably said means to monitor the temperature of the hot gas ismounted closer to the lower temperature zone than to the highertemperature zone.

The configuration is preferably such that the hot gas is directeddownwardly from the burner assembly through the heat exchanger assemblyto traverse the higher temperature zone and then the lower temperaturezone. Means is advantageously provided under the heat exchanger assemblyfor collecting condensate that forms in said lower temperature zone.

In a second aspect, the invention provides a heat exchanger apparatus,including:

-   -   a heat exchange module having a plurality of heat exchange        elements, extending across a passage, the passage being arranged        to convey a first fluid past and about the heat exchanger        elements; and    -   one or more return headers adapted to be selectively mounted to        said module either for directing a second fluid in turn through        any adjacent pair of heat exchange elements, or for directing a        second fluid from one heat exchange element of said module to a        heat exchange element of a similar module when said module is        coupled to said similar module.

The return header can have a separate sealing engagement with each bankof tubes. Most preferably, the return header has a separate sealingengagement with each tube.

The banks of tubes are preferably relatively displaced along saidpassage. Advantageously, first and second banks of tubes have theirtubes formed in materials that respectively suit a lower temperatureoperation and higher temperature operation. A suitable material for thetubes of the lower temperature bank is aluminium sheathed stainlesssteel while a suitable material for the tubes of the higher temperaturebank is cupronickel. Copper is another material suitable for the tubesof the higher temperature bank(s).

Preferably there are a plurality of modules in coupled relation and aplurality of like return headers for interconnecting banks of tubeswithin the modules and interconnecting modules. Advantageously eachmodule has only two banks of tubes.

In an embodiment, there are two of said modules and three returnheaders, two mounted for directing the second fluid from one of thebanks of tubes to the other in the respective pair, and a third fordirecting the second fluid from a second bank of one module to a firstbank of the other module. Preferably, in this embodiment, the successivespacings of the four banks of tubes along said passage are substantiallyequal, and the tubes of the respective pairs of banks are formed inmaterials that respectively suit a lower temperature operation andhigher temperature operation.

In a particularly useful application, a heat exchanger apparatusaccording to the second aspect of the invention is employed as the heatexchanger assembly of the first aspect of the invention.

A third aspect of the invention relates to the condensate which is aby-product from some types of pool heater and indeed from one or moreembodiments of the first and second aspects of the present invention.More particularly, in its third aspect, the invention provides a waterheater for heating water, including:

-   -   a burner assembly for generating a flow of hot gas;    -   a heat exchanger assembly arranged to receive said flow of hot        gas for transferring heat from the gas to water flowing therein;    -   ducting to conduct said water to and from said heat exchanger        assembly;    -   means to collect condensate produced from condensation of said        gas in said heat exchanger assembly; and    -   a condensate duct to direct said condensate into said water for        chemically treating said water.

The condensate will typically be slightly acidic, i.e. have a pHslightly less than 7, and said chemical treatment may comprise pHadjustment. In one embodiment of the third aspect of the invention, thecondensate is directed into the heated stream of water immediatelydownstream of the heat exchanger assembly, and for this purpose saidcondensate duct may include a venturi at which the condensate is drawninto the heated water stream. A pump may be arranged to receivecondensate from the means to collect condensate and drive the condensatethrough the condensate duct. In an alternative embodiment, thecondensate is stored and said condensate duct forms part of dosingapparatus for selectively directing metered amounts of condensate intothe pool water at any suitable location.

In a further alternative embodiment condensate may be directed into thewater with the aid of a suction tee.

Advantageously the condensate duct may be arranged to direct thecondensate into the water upstream of a pump arranged to drive saidwater through said water heater.

The means to collect condensate may comprise a tray or housing base in awater heater according to the first aspect of the invention or in orbelow a heat exchanger apparatus according to the second aspect of theinvention.

In its third aspect, the invention further provides a method ofchemically treating water in a pool comprising adding to the watercondensate collected from a heat exchanger assembly of a water heaterthrough which the pool water is circulated and heated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a pool heater according to an embodimentof the invention, as viewed without its exterior decorative housing;

FIG. 2 is a rear view of the pool heater depicted in FIG. 1 with someparts omitted for a better view and with the condensate venturiadditionally shown in place;

FIG. 3 is a vertical, generally central cross-section of the pool heaterdepicted in FIGS. 1 and 2, with most of the heat exchanger tubes omittedfor the purpose of illustration;

FIGS. 4 and 5 are different perspective views of the heat exchangerassembly;

FIG. 6 is a view of the heat exchanger assembly, and with many of theupper bank of tubes omitted;

FIG. 7 is a plan view of the heat exchanger assembly;

FIG. 8 is a cross-section on the line 8-8 in FIG. 7;

FIG. 9 is a simplified schematic diagram of the burner control loopincorporating a temperature sensor in the heat exchanger assembly;

FIGS. 10 and 11 are respectively a perspective view and an end elevationof a larger heat exchanger assembly with four banks of tubes;

FIG. 12 is a rear view one embodiment of the third aspect of theinvention entailing recycling of condensate collected from the heatexchanger assembly;

FIG. 13 is a fragmentary axial cross-section view of the condensateventuri forming part of the embodiment of FIG. 12;

FIG. 14 is a rear view of an alternative embodiment of the condensaterecycling concept;

FIG. 15A is a perspective view of an embodiment of the return header;

FIGS. 15B and 15C are perspective cut away views of the header of FIG.15A;

FIG. 16 is a perspective view an embodiment of the tray;

FIG. 17 is a rear view of a further alternative embodiment of thecondensate recycling concept; and

FIG. 18 is a rear view of a further alternative embodiment of thecondensate recycling concept.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The illustrated pool heater 10 is a stacked assembly of four principalcomponents: a tray 80 over which is fitted a heat exchanger assembly 20on which is mounted a firebox 50, which is in turn capped by a fan unit60 that includes a controller 70 with an external interface 72 and a lid62 that is removable for access. Tray 80 sits in plastic base 12.

Tray 80 is a unitary casting and, as will be further explained below,serves as a condensate collection tray. Tray 80 sealingly communicateswith a flue 82 that extends upwardly behind the heat exchanger assembly20, firebox 50 and fan unit 60 to a flue outlet 83.

In situ and in operation, pool water is circulated by a separate pumpinstallation to a water intake port 22 and recovered from outlet 23. Afan 64 within fan unit 60 draws in a correctly proportioned combustiblemixture of gas (delivered via line 65) and air, and delivers the mixtureto a gas burner 52 at the top of firebox 50. The gas burner 52 and thefan unit 60 together form a burner assembly 54 (FIG. 3) that generates adownwardly directed flow of hot gas. This flow is received by the heatexchanger assembly 20 where heat is transferred from the hot gas to poolwater flowing therein. The burner 52 is of the premix type and includesa knitted mesh. Below the heat exchanger 20, the hot gas is guidedlaterally by the shaped tray 80 to the base of flue 82 and thence up theflue.

Fan 64 constitutes a means that is adjustable to vary the volume of gasand air directed to the burner 52 and so determines the heat output ofthe burner. The fan 64 and gas burner 52 together constitute a burnerassembly for generating a flow of hot gas.

The construction of heat exchanger assembly 20 is detailed in FIGS. 4 to8. A simple box 24 of front, rear and side flanged plates 24 a, 24 b, 24c provides a suitable chassis. Side plates 24 b, 24 c have two rows ofapertures 25 a and 25 b that communicate with the interior of heatexchange tubes 27, 29 arrange in respective lower and upper banks 26,28. At one side plate 24 b, there are fitted respective inlet and outletheaders 30, 32 that define a manifold space respectively communicatingthe lower and upper apertures 25 and therefore the lower tubes 27 andupper tubes 29 to water inlet ports 22, 23. At the other side plate 24c, there is a return header 34, a suitably profiled moulding thatdefines a manifold space for communicating the lower of apertures 25 awith the upper row 25 b. Vanes 31 are placed between the tubes 27, 29 todeflect the gas flow and improve the heat transference to tubes 27, 29.

A convenient method for assembling the module like box 24 involvesforming side plates 24 b and 24 c with apertures 25 a, 25 b beingslightly oversized, e.g. 0.1 mm, to receive the tubes 27, 29. Tubes 27,29 are inserted into apertures 25 a, 25 b and a rotary swage used toexpand the tubes to form an interference with sideplates 24 b, 24 c. Anadvantage of certain embodiments of the second aspect of the inventionis that large heat exchangers can be economically built up of severalmodules each having two rows. The tubes of a two row module are easilygripped to prevent rotation during the rotary swaging operation.

Although modules having two rows defining U-shaped flow paths areillustrated, other arrangements are possible. For example, a modulehaving three rows defining an S-shape flow path is an option.

Return header 34 is shown in more detail in FIGS. 15A to 15C. Apertures35 are connected by cavity 37 and include a recess to receive a sealingwasher (not shown) to sealingly engage with individual tubes 27, 29. Theregular spacing of bolt locations 36 allows the header to securely pressthe sealing washers with less risk of leakage due to warping of theheader. This advantageously allows for a cheaper moulded plastic(instead of cast metal) construction.

It will be seen that because the cooler water traverses the lower bankof tubes 27 and then the upper bank of tubes 29, the lower bank 26constitutes a lower temperature zone 100 of the heat exchanger assemblyand the upper bank 28 constitutes a higher temperature zone 110.Accordingly, the respective banks of tubes are formed of differingmaterials: the lower tubes 27 are aluminium-sheathed stainless steeltubes, while the upper tubes 29 are of cupronickel alloy. It will beseen that the descending flow of hot gas will pass through and abouttubes 29 first and then, in a cooler state, through and about tubes 27.

The cupronickel tubes 29 are effective heat exchange elements at highertemperatures but are highly susceptible to corrosion by any condensatethat forms on them in the gas flow, while the aluminium/stainless steeltubes 27 are resistant to condensate corrosion but degrade at relativelylow elevated temperatures. Accordingly, in accordance with the firstaspect of the invention, the temperature profile in the gas streamacross the heat exchanger is managed to accommodate thesecharacteristics. Temperature sensor 40 (FIG. 8) is located on thevertically centred plane of the heat exchanger assembly inwardly fromside panel 24 c between the respective banks 26, 28 of heat exchangetubes. The sensor output is delivered to controller 70 which adjusts thefan 64 to determine the heat output of burner 52 in response to variousinputs including sensor 40. Other inputs may include a desired watertemperature manually entered at interface 72, and actual watertemperature measured by sensor 73 on inlet header 32. A suitablecontroller is a Genus PCB controller.

A diagram of the main elements of the burner control loop is presentedin FIG. 9.

In particular, controller 70 is responsive to temperature sensor 40(monitoring the temperature at its location in the heat exchangerassembly), to operate fan 64 so as to modulate the heat output of burner52, whereby to maintain the monitored temperature at sensor 40 within apredetermined set point range. This range is between a minimum selectedso that the gas temperature in the higher temperature zone 110 remainsabove the dew point condensation temperature, and a maximum isdetermined so that, inter alia, the temperature of the gas deliveredinto the lower temperature zone 100 is not so high as to damagealuminium/stainless steel tubes 27. In the former case, condensation ofvapour from the gas is substantially prevented or minimised in thehigher temperature zone 110 of the heat exchanger assembly.

FIGS. 10 and 11 illustrate the manner in which the heat exchangerconstruction is readily adaptable to provide higher capacity heatexchangers. In the heat exchanger of FIGS. 4 to 8, the side plates 24 b,24 c and tubes 27, 29 constitute a heat exchange module 105. By formingthe box chassis 24 from two of these modules 105 a, 105 b fixed betweenfront and rear plates 24 a of double height, comprising four banks 126,128 of tubes 127, 129 can be provided. In this case, the lower andhigher temperature zones are defined by the respective modules 105 a,105 b.

This modular approach to enlarging the capacity of the heat exchangermeans that three identical return headers 34 can be utilised asillustrated to direct water between the tubes of the two lower banks andbetween the tubes of the two upper banks, and also, on the other side ofthe box chassis 24, from the tubes of the lower, aluminium/stainlesssteel tubes to the upper cupronickel tubes. The inlet and outlet headers30, 32 are identical to the inlet headers 30, 32 depicted in FIGS. 4 to6.

The illustrated configuration of water heater, including the two-stageheat exchanger configuration and the control of burner heat output inresponse to monitoring of the temperature in the heat exchanger,together result in a pool heater system of significantly higherefficiency than the earlier described conventional arrangements. Fullvolume water flow, say up to 400 L/min is maintained without periodicbypassing and burner output is matched with the desired set point gastemperature range in the heat exchanger. Condensation is accepted andproperly managed by employing a two-stage heat exchanger in which thematerials of the heat exchange elements are selected to suit therespective higher and lower temperature zones.

The third aspect of the invention is concerned with the novel usage forthe condensate collected in tray 80 which is depicted in more detail inFIG. 16. The concept is that this condensate, which contains traces ofcombustion by-products and is thereby slightly acidic, is recycled tothe pool water as an effective chemical treatment. There are variousways in which this can be done. In the first (illustrated in FIGS. 12and 13), a suitably dimensioned conduit 84 communicates the sump 81 viaoutlet 89 of tray 80 (see FIG. 16) with the feed port 86 of a venturisuction device 87 fitted within water outlet port 23. Conduit 84includes solenoid valve 85 for selectively determining when condensatecan flow to the venturi. The solenoid valve 85 is used to close conduit84 when the heater 10 and pump are not in use to prevent water flowingthrough conduit 84 to tray 80. A suitable construction for the venturi87 is illustrated in FIG. 13: it will be seen that the feed port 86 atthe end of conduit 84 communicates with a chamber 87 from which anaperture 88 opens into the neck of the venturi.

A float sensor (not shown) may be associated with the sump 81 to detectblockage of the outlet 89 or conduit 84.

In the alternative condensate recycling arrangement depicted in FIG. 14,a drain hose 90 from sump 81 conveys the condensate to a storagereservoir 92 from which the condensate is selectively drawn via a tube93 by a dosing unit 94 for delivery at an insertion point 98 in the poolwater return pipe 96 downstream of water heater 10.

In a further alternative embodiment illustrated in FIG. 17 the drainhose 90 is selectively closed by the solenoid valve 201. Downstreamalong the drain hose 90 from the solenoid valve 201 is the condensatepump 202. The condensate pump 202 may be activated and the solenoidvalve 201 opened to drive condensate collected in the tray 80 throughthe pump discharge line 203. The pump discharge line 203 extends fromthe pump 202 to a suction tee 204 positioned along the return pipe 96downstream of water heater 10. The use of the solenoid valve 201prevents water back feeding from the return pipe 96 to the tray 80.

FIG. 18 illustrates a further alternative embodiment which includes acollector reservoir 210 interposed between the tray 80 and the solenoidvalve 201 along the drain hose 90 to store condensate. A condensatesuction line 211 extends from the solenoid valve 201 to a suction tee212. The suction tee 212 is fitted to an inlet line 214 for supplyingwater to the water intake port 22. A pump 213 is positioned along theinlet line 214 to draw water from the swimming pool and drive the waterthrough the heat exchanger 20 (the water is in turn returned to the poolvia the return pipe 96). The suction tee 212 is thereby in a lowpressure region upstream of the pump 213 and in addition is configuredto create some venturi effect so that condensate may be drawn into theinlet pipe 214 when the solenoid valve 201 is open.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.All of these different combinations constitute various alternativeaspects of the invention.

1. A water heater for heating water, including: a burner assembly forgenerating a flow of hot gas, which burner assembly includes a gasburner and means adjustable to determine a heat output of the burnerassembly; a heat exchanger assembly for transferring heat from gas towater flowing therein, wherein the heat exchanger assembly has a highertemperature zone and a lower temperature zone, the water heater beingarranged to convey the flow of hot gas to the higher temperature zoneand in turn to the lower temperature zone; ducting to conduct saidflowing water to and from said heat exchanger assembly; means to monitorthe temperature of the hot gas intermediate the higher temperature zoneand the lower temperature zone; and control means responsive to saidtemperature monitoring means to modulate the heat output of the burnerassembly whereby to maintain the monitored temperature within apredetermined range so as to substantially prevent or minimisecondensation of vapour from the hot gas in the higher temperature zone.2. A water heater according to claim 1 wherein use the monitoredtemperature is maintained within a pre-determined range without reducingthe volume of water flowing through the heat exchanger.
 3. A waterheater according to claim 1 wherein said control means is operable tominimize said monitored temperature.
 4. A water heater according toclaim 1 wherein said means to monitor the temperature of the hot gas ismounted closer to the lower temperature zone than to the highertemperature zone.
 5. A water heater according to claim 1 wherein thelower temperature zone and the higher temperature zone each have tubesfor carrying water through the flow of hot gas, the tubes of the lowertemperature zone and the higher temperature zone being formed ofmaterials that respectively suit lower temperature operation and highertemperature operation.
 6. A water heater according to claim 5 whereinthe tubes of the lower temperature zone are formed of aluminium sheathedstainless steel and the tubes of the higher temperature zone are formedof cupronickel.
 7. A water heater according to claim 5 wherein the tubesof the higher temperature zone are formed of copper.
 8. A water heateraccording to claim 1 configured for the flow of hot gas to traveldownwardly from the burner assembly through the higher temperature zoneand in turn through the lower temperature zone.
 9. A water heateraccording to claim 1 having means to collect condensate produced fromcondensation of gas in said lower temperature zone.
 10. A water heateraccording to claim 9 including a condensate duct arranged to direct saidcondensate into the water for chemically treating the water.
 11. A waterheater according to claim 10 wherein the condensate duct is arranged todirect said condensate into the ducting from the heat exchangerassembly.
 12. A water heater according to claim 10 wherein thecondensate duct is arranged to direct condensate into the water upstreamof a pump arranged to drive water through the water heater.
 13. A waterheater according to claim 10 including a venturi to draw said condensateinto the water.
 14. A water heater according to claim 10 including asuction tee to draw the condensate into the water.
 15. A water heateraccording to claim 10 including a pump arranged to receive condensatefrom said means to collect condensate and drive said condensate throughsaid condensate duct.
 16. A water heater according to claim 9 includinga dosing apparatus for storing and selectively directing metered amountsof condensate into the water at any suitable location.
 17. A heatexchanger apparatus, including: a heat exchange module having aplurality of heat exchange elements extending across a passage, thepassage being arranged to convey a first fluid past and about the heatexchanger elements; and one or more return headers adapted to beselectively mounted to said module either for directing a second fluidin turn through any adjacent pair of heat exchange elements, or fordirecting a second fluid from one heat exchange element of said moduleto a heat exchange element of a similar module when said module iscoupled to said similar module.
 18. A heat exchanger apparatus accordingto claim 17 wherein each heat exchange element is a bank of tubes.
 19. Aheat exchanger apparatus according to claim 18 wherein the return headerhas a separate sealing engagement with each tube.
 20. A heat exchangerapparatus according to claim 18 wherein the banks of tubes arerelatively displaced along said passage.
 21. A heat exchanger apparatusaccording to claim 18 wherein the banks of tubes respectively form oneor more lower temperature banks of tubes and one or more highertemperature banks of tubes having their tubes formed in differingmaterials that respectively suit lower temperature operation and highertemperature operation.
 22. A heat exchanger apparatus according to claim21 wherein the tubes of the one or more lower temperature banks of tubesare formed of aluminium sheathed stainless steel and the tubes of theone or more higher temperature banks of tubes are formed of cupronickel.23. A heat exchanger apparatus according to claim 21 wherein the tubesof the one or more higher temperature banks of tubes are formed ofcopper.
 24. A heat exchanger apparatus according to claim 18 having aplurality of the modules in coupled relation and a plurality of likereturn headers interconnecting banks of tubes within the modules andinterconnecting modules.
 25. A heat exchanger apparatus according toclaim 18 to wherein each module has only two banks of tubes.
 26. A heatexchanger apparatus according to claim 25 having two of said modules andthree of said return headers, two of the return headers beingrespectively mounted for directing the second fluid between the banks oftubes within a respective module, and a third return header fordirecting the second fluid from a second bank of one module to a firstbank of the other module.
 27. A heat exchanger apparatus according toclaim 26 wherein the successive spacings of the banks of tubes alongsaid passage are substantially equal, and the tubes one of the modulesand the tubes of the other of the module being formed in materials thatrespectively suit a lower temperature operation and higher temperatureoperation.
 28. A heat exchanger apparatus according to claim 18 whereinthe first fluid is hot gas from a burner assembly and the second fluidis water.
 29. A water heater according to claim 1 wherein the heatexchanger assembly includes: a heat exchange module having banks oftubes extending across a passage, the passage being arranged to conveythe hot gas past and about the bank of tubes; and one or more returnheaders adapted to be selectively mounted to said module either fordirecting the water in turn through any adjacent pair of tube banks, orfor directing the water from one bank of tubes of said module to a bankof tubes of a similar module when said module is coupled to said similarmodule.
 30. A water heater for heating water, including: a burnerassembly for generating a flow of hot gas; a heat exchanger assemblyarranged to receive said flow of hot gas for transferring heat from thegas to water flowing therein; ducting to conduct said water to and fromsaid heat exchanger assembly; means to collect condensate produced fromcondensation of said gas in said heat exchanger assembly; and acondensate duct to direct said condensate into said water for chemicallytreating said water.
 31. A water heater according to claim 30 whereinthe condensate duct is arranged to direct said condensate into theducting from the heat exchanger assembly.
 32. A water heater accordingto claim 30 wherein the condensate duct is arranged to direct condensateinto the flowing water upstream of a pump arranged to drive waterthrough said water heater.
 33. A water heater according to claim 30including a venturi to draw said condensate into said water.
 34. A waterheater according to claim 30 including a suction tee to draw saidcondensate into said water.
 35. A water heater according to claim 30including a pump arranged to receive condensate from said means tocollect condensate and drive said condensate through said condensateduct.
 36. A water heater according to claim 30 including a dosingapparatus along said condensate duct for storing and selectivelydirecting metered amounts of condensate into the water at any suitablelocation.
 37. A method of chemically treating water in a pool comprisingadding to the water condensate collected from a heat exchanger assemblyof a water heater through which the pool water is circulated and heated.38. A water heater according to claim 2 wherein said control means isoperable to minimize said monitored temperature.
 39. A water heateraccording to claim 38 wherein the lower temperature zone and the highertemperature zone each have tubes for carrying water through the flow ofhot gas, the tubes of the lower temperature zone and the highertemperature zone being formed of materials that respectively suit lowertemperature operation and higher temperature operation.